Abstract: GMZ bentonite is the first selection of buffer/backfill material for geological disposal repository of high-level radioactive waste in china. It will work under high temperature and high pressure for a long time. In order to investigate the strength characteristic of GMZ bentonite under high temperature and high pressure, 81 undrained triaxial shearing tests were conducted by using the high-temperature and high-pressure triaxial apparatus, and the effect of dry density, confining pressure, temperature and water content on its strength characteristic were systematically analyzed. The result shows that (1) confining pressure and dry density have a great influence on the failure forms of GMZ bentonite; (2) The strength of GMZ bentonite decrease with the increase of water content; The influence of temperature and dry density on its strength is complex, the position of deviator stress-axial strain curves are rising with the increase of temperature in low dry density, but it’s on the contrary in high dry density; (3) The formula of cohesion and internal friction angle with the changing of dry density, water content and temperature of GMZ bentonite are established, respectively. The research results of this paper provide a scientific basis for analyzing the thermos-hydro-mechanical coupling characteristics of buffer materials.